What Causes Tides
In Ocean
Overview
We often hear weather man, on the TV, talking about the timings of high tide and low tide at a particular shoreline. Have you ever wondered what a tide is? What is the difference between a tide and a wave? What causes tides? And what produces waves in oceans?
Before getting into the cause of tides, first we need to understand the difference between tides and waves.
Tides vs Waves
It is important to distinguish between tides and waves. Tide refers to the change in level of water in the ocean. Tides in the ocean causes the water level to change along the shorelines. However, the changing water level along many shores may not be noticeable. But some shores exhibit dramatic changes in water level. For instance, the Bay of Fundy in Canada, has the largest change, in the world, between low tides and high tides.
Whereas waves refers to the horizontal movement of energy through water. The primary cause for the waves in the ocean is wind. Wind currents, due to the heating of the atmosphere by the Sun’s radiation, cause surface waves in deep ocean that travel to all the shores.
What causes tides? Why does the water level change in oceans? The simple answer is it is due to the gravity of our Moon and our Sun. Especially, the Moon plays a dominant role in causing tides in the ocean.
Let us try to understand how the Moon’s gravitational effect causes low and high tides in the ocean.
What Causes Tides
What causes tides (high and low tides) in oceans on Earth? The simple answer is our astronomical neighbor, the Moon, is responsible for it. Of course, our Sun too contributes to it, but the Moon plays a bigger role because of its proximity to Earth.
As water in the ocean is mobile and flexible, Moon’s gravity causes a bulge on the huge body of water not only on the side nearer to it, but also on the opposite side of Earth. This is because of the tidal force caused by the gravitational pull of the Moon. The near side of the Earth experiences more gravitational force than the far side of the Earth. Consequently, with respect to the center of the Earth, the near side bulges towards the moon while the far side bulges away in the opposite direction.
The results of these bulges are high tides on the two sides in line with the moon and lows tides on the other two sides of the Earth which are perpendicular to the line connecting the centers of Earth and the Moon, as the picture above shows.
As the Earth rotates on its axis once in 24 hours, the location of the high tides stay (remember that the Moon revolves around Earth slowly taking about 28 days), while we earthlings move towards the low tide in about 6 hours. And then again, we move towards the high tide on the other side in the next 6 hours. Reaching the low tide again at the end of the next 6 hours.
That is the reason shorelines experience two low tides and two high tides every day. However, the level of change is not the same along different shorelines. It depends on characteristics of a shoreline.
Spring Vs Neap Tides
Spring Tide
Spring tides are the high tides that occur on full moon days or on new moon days.
You may wonder why the moon should only cause this tidal effect. What about the Sun, which is much, much bigger than the Moon?
Yes, the Sun’s gravity also causes tidal bulge on the oceans on the Earth. However, because of the vast distance between the Earth and the Sun, the bulge that the Sun causes is only half of that of the Moon.
On a full moon or a new moon day, the Moon, the Earth and the Sun come aligned in a line. So, the tidal effects, from the Moon and the Sun, add up. The net result is a much larger high tide bulge as the picture above illustrates.
This explains why we see much bigger tidal range (the difference between high and low levels) on a full moon day or on a new moon day.
Neap Tide
What happens when the Moon is at the end of the first quarter or at the beginning of the last quarter?
During this time, the Moon and the Sun are in perpendicular direction. Consequently, the high tide of the Moon gets reduced as the picture above shows. There will be the least difference between the low tides and the high tides.
So, the neap tides occur at the end of first quarter and the beginning of the last quarter of the Moon.
Frictional Force
If you closely look at the picture above, you may notice that the high bulges are NOT perfectly aligned with the Moon. This happens because of the friction between the surface of the earth and water. The rotating earth moves the bulge slightly away from the direction of the moon.
Incidentally, it is the same friction that is responsible for slowing the rotation of the earth. That is the reason we add a leap second to our atomic clocks once in a while.
Key Takeaways
- Tides are vertical changes in levels of the ocean whereas waves are the horizontal movement of energy through water. Primary factor for waves is wind.
- Gravitational force of the Moon causes tides as it causes bulges in the ocean.
- Our Sun too causes tidal bulge. But the bulge is much less as it is far away from the Earth, though it is much more massive compared with the Moon.
- Spring tide: it occurs on a full moon or on a new moon day as the Moon, The Earth and the Sun come aligned in a line. The high tides of the Moon and the Sun get reinforced to produce higher tidal range.
- Neap tide: it occurs during the period when the Moon is either at the end of the first quarter or at the beginning of the last quarter. Because the Sun and the Moon are in perpendicular direction, the tidal bulges from the Moon get reduced resulting in the least difference between the low tide and high tide.
- Friction: As the Earth rotates every day, the friction between Earth’s surface and water makes the bulges not aligned with the Moon. It is the same friction that slows down the rotation of the Earth tiny bit which requires a leap second to be added to our atomic clocks – the timekeepers.